The "definitive requirement" I suppose is that a bus is a transmission
line and is modelled like one. That means that it should
(in this instance) be terminated on each end. "End" is unique here, as
one could have two backplanes connected by a ribbon
cable. This was the case in early systems where you could not put
enough stuff on a single card and had lots of cards in a
backplane, and therefore needed to go to a second (or third) backplane.
Some of the third-party backplanes had a method
for termination right on the backplane, so you would not lose a slot.
This was for the "tail" end. I recollect that every CPU had
terminators on it, as it was supposed to be the first module (or was the
top of each backplane terminated? don't remember)
Each backplane/extension cable was usually terminated when there were
multiple backplanes as well (as I remember).
Some of those termination boards had bootstraps on them as well. That
is why there are several versions of the terminator
board (term only, term and ribbon cable plug, term/boot, etc)

In reality, I rarely paid attention to termination when using a single
backplane. I have also made some configurations where
the CPU was not first, nor was it in the first slot. I have had memory
before the CPU. (doesn't need to keep the
interrupt chain together, so you can get away with it). I have moved
everything down in the backplane because of some
bad slots and couldn't afford a new backplane. Yes, I have regularly
broken the rules. Things still worked.

When troubleshooting, I try to use the least amount of modules as
possible in a system. If that means pulling the far-end
termination, so be it. If it means putting the boards in the middle
of the backplane if you suspect a bad slot, well, go for
it. But you have to be very aware of the configuration of the
backplane, if it is ABCD or ABAB (straight or serepentine)
and if you need interrupts (almost always do), etc, if you are going to
stray from the published configurations.

All if this is based on my Q-bus work, it's been a very long time that I
worked on Unibus, but I'll bet the transmission
line stuff holds.

A terminator is a device that prevents "ringing", that is, reflecting
the signal off the end of the bus and back into the
bus/backplane, creating "ghosts". A signal will reflect back into the
bus at the end of the backplane when it sees
an impedance mismatch (now getting very technical), which is what
happens when it goes from the backplane and out
into thin air (end of bus). A resistor/divider network at the end
minimizes this. If there is no terminator and the ghost
is not large enough, you can "get away" without a terminator.

Too many terminators is another issue. If you put too many into a
system, then the bus driver chips cannot put enough
power into the bus to drive from low to high (or high to low, depending
if the bus is active high or not) and the signal
will not "transmit" strongly enough for the other boards to hear it.

As usual, this is all from memory, I hope I didn't make the waters muddier.

Joe Heck

Vintage Computer Festival wrote:

>So what's the definitive requirement for terminating QBus? I get various
>non-answers from various people and it's quite annoying.
>
>How does one properly terminate Qbus?
>
>
>
Received on Tue Feb 08 2005 - 09:35:09 GMT